A Case of Subcutaneous Neuroma Presenting With Intractable Pain and Allodynia Over the Anteromedial Aspect of the Knee

Abstract

With the exception of interdigital neuromas, cutaneous neuromas are relatively rare and often present a diagnostic challenge. We describe a case of a 30-year-old man with a 9-month history of intractable pain and touch allodynia on the medial side of his proximal left leg. Although the exact focus of the pain and allodynia was initially difficult to detect, a subsequent thorough physical examination revealed touch allodynia in a 1×1-cm area overlying the proximal tibia, immediately below the left patella. Ultrasonography of this site with a 7.5-MHz linear probe showed a 2×4-mm round hypoechoic mass with smooth margins that was suspected to be a neuroma arising from the infrapatellar branch of the saphenous nerve. An excisional biopsy was then performed, the pathology of which revealed perineurial thickening, inflammatory cells in the perineurium, and neovascularization, consistent with neuroma. All symptoms disappeared immediately after an excisional biopsy. The diagnosis of cutaneous neuroma should be considered in all patients who have intractable pain and allodynia at unusual locations and in atypical patterns. In addition, ultrasonography can prove very useful in the detection of small cutaneous neuromas if the site of symptoms can be precisely localized.